In an impact tool, a rotation striking mechanism is driven by a motor as a driving source to provide rotation and striking to an anvil, thereby intermittently transmitting rotation striking power to a tip tool for performing operation, such as screwing. As a motor, a brushless DC motor is widely used. The brushless DC motor is, for example, a DC (direct current) motor with no brush (brush for commutation). Coils (windings) are used on the stator side, magnets (permanent magnets) are used on the rotor side, and a rotor is rotated as the electric power driven by an inverter circuit is sequentially applied to predetermined coils. The inverter circuit is constructed using an FET (field effect transistor), and a high-capacity output transistor such as an IGBT (insulated gate bipolar transistor), and is driven by a large current. The brushless DC motor has excellent torque characteristics as compared with a DC motor with a brush, and is able to fasten a screw, a bolt, etc. to a base member with a stronger force.
JP-2009-072888-A discloses an impact tool using the brushless DC motor. In JP-2009-072888-A, the impact tool has a continuous rotation type impact mechanism. When torque is given to a spindle via a power transmission mechanism (speed-reduction mechanism), a hammer which movably engages in the direction of a rotary shaft of the spindle rotates, and an anvil which abuts on the hammer is rotated. The hammer and the anvil have two hammer convex portions (striking portions) which are respectively arranged symmetrically to each other at two places on a rotation plane, these convex portions are at positions where the gears mesh with each other in a rotation direction, and rotation striking power is transmitted by meshing between the convex portions. The hammer is made axially slidable with respect to the spindle in a ring region surrounding the spindle, and an inner peripheral surface of the hammer includes an inverted V-shaped (substantially triangular) cam groove. A V-shaped cam groove is axially provided in an outer peripheral surface of the spindle, and the hammer rotates via balls (steel balls) inserted between the cam groove and the inner peripheral cam groove of the hammer.
In the conventional power transmission mechanism, the spindle and the hammer are held via the balls arranged in the cam groove, and the hammer is constructed so as to be able to retreat axially rearward with respect to the spindle by the spring arranged at the rear end thereof. As a result, the number of parts of the spindle and the hammer increases, high attaching accuracy between the spindle and the hammer is required, thereby increasing the manufacturing cost.
Meanwhile, in the impact tool of the conventional technique, in order to perform a control so as not to operate the impact mechanism (that is, in order that striking does not occur), for example, a mechanism for controlling a retreat operation of the hammer is required. The impact tool of JP-2009-072888-A cannot be used in a so-called drill mode. Further, even if a drill mode is realized (even if a retreat operation of the hammer is controlled), in order to realize even the clutch operation of interrupting power transmission when a given fastening torque is achieved, it is necessary to provide a clutch mechanism separately, and realizing the drill mode and the drill mode with a clutch in the impact tool leads to cost increase.
Further, in JP-2009-072888-A, the driving electric power to be supplied to the motor is constant irrespective of the load state of a tip tool during the striking by the hammer. Accordingly, striking is performed with a high fastening torque even in the state of light load. Asa result, excessive electric power is supplied to the motor, and useless power consumption occurs. And, a so-called coming-out phenomenon occurs where a screw advances excessively during screwing as striking is performed with a high fastening torque, and the tip tool is separated from a screw head.